Ethylbenzene is a key raw material in the production of styrene and is produced by the reaction of ethylene and benzene in the presence of an acid alkylation catalyst. Older ethylbenzene production plants, those typically built before 1980, used AlCl3 or BF3 as the acidic alkylation catalyst. Plants built after 1980 have in general used zeolite-based acidic catalysts as the alkylation catalyst.
Commercial ethylbenzene manufacturing processes typically require the use of concentrate ethylene that has a purity exceeding 80 mol. %. For example, a polymer grade ethylene has a purity exceeding 99 mol. % ethylene. However, the purification of ethylene streams to attain chemical or polymer grade is a costly process and hence there is considerable interest in developing processes that can operate with lower grade or dilute ethylene streams. One source of a dilute ethylene stream is the off gas from the fluid catalytic cracking or steam-cracking unit of a petroleum refinery. The dilute ethylene stream, after removal of reactive impurities, such as propylene, typically contains about 10-80 mol. % ethylene, with the remainder being ethane, hydrogen, methane, and/or benzene.
Three types of ethylation reactor systems are used for producing ethylbenzene, namely, vapor phase reactor systems, liquid phase reactor systems, and mixed phase reactor systems.
In vapor-phase reactor systems, the ethylation reaction of benzene and ethylene is carried out at a temperature of about 350 to 450° C. and a pressure of 690-3534 KPa-a (6-35 kg/cm2-g) in multiple fixed beds of zeolite catalyst. Ethylene exothermically reacts with benzene to form ethylbenzene, although undesirable reactions also occur. About 15 mol. % of the ethylbenzene formed further reacts with ethylene to form di-ethylbenzene isomers (DEB), triethylbenzene isomers (TEB) and heavier aromatic products. All these undesirable reaction products are commonly referred as polyethylated benzenes (PEBs).
By way of example, vapor phase ethylation of benzene over the crystalline aluminosilicate zeolite ZSM-5 is disclosed in U.S. Pat. No. 3,751,504 (Keown et al.), U.S. Pat. No. 3,751,506 (Burress), and U.S. Pat. No. 3,755,483 (Burress).
In most cases, vapor phase ethylation systems use polymer grade ethylene feeds. Moreover, although commercial vapor phase processes employing dilute ethylene feeds have been built and are currently in operation, the investment costs associated with these processes is high.
In recent years the trend in industry has been to shift away from vapor phase reactors to liquid phase reactors. Liquid phase reactors operate at a temperature of about 150-280° C., which is below the critical temperature of benzene (290° C.). The rate of the ethylation reaction is lower compared with the vapor phase, but the lower design temperature of the liquid phase reaction usually economically compensates for the negatives associated with the higher catalyst volume.
Liquid phase ethylation of benzene using zeolite beta as the catalyst is disclosed in U.S. Pat. No. 4,891,458 and European Patent Publication Nos. 0432814 and 0629549. More recently it has been disclosed that MCM-22 and its structural analogues have utility in these alkylation/transalkylation reactions, for example, U.S. Pat. No. 4,992,606 (MCM-22), U.S. Pat. No. 5,258,565 (MCM-36), U.S. Pat. No. 5,371,310 (MCM-49), U.S. Pat. No. 5,453,554 (MCM-56), U.S. Pat. No. 5,149,894 (SSZ-25); U.S. Pat. No. 6,077,498 (ITQ-1); International Patent Publication Nos. WO97/17290 and WO01/21562 (ITQ-2).
Commercial liquid phase ethylbenzene plants normally employ polymer grade ethylene. Moreover, although plants can be designed to accept ethylene streams containing up to 30 mol. % ethane by increasing the operating pressure, the costs associated with the design and operation of these plants have proven to be significant.
Technology has also been developed for the production of ethylbenzene in a mixed phase using reactive distillation. Such a process is described in U.S. Pat. No. 5,476,978. Mixed phase processes can be used with dilute ethylene streams since the reaction temperature of the ethylation reactor is below the dew point of the dilute ethylene/benzene mixture, but above the bubble point. The diluents of the ethylene feed, ethane, methane and hydrogen, remain essentially in the vapor phase. The benzene in the reactor is split between vapor phase and liquid phase, and the ethylbenzene and PEB reaction products remain essentially in the liquid phase.
U.S. Pat. No. 6,252,126 discloses a mixed phase process for producing ethylbenzene by reaction of a dilute ethylene stream containing 3 to 50 mol. % ethylene with a benzene stream containing 75 to 100 wt. % benzene. The reaction is conducted in an isothermal ethylation section of a reactor, which also includes a benzene stripping section, where the unreacted benzene is thermally stripped from the ethylation products. Integrated, countercurrent vapor and liquid traffic is maintained between the ethylation section and the benzene stripping section.
U.S. patent application Ser. No. 10/252,767 discloses a process for the production of ethylbenzene by reacting benzene with a dilute ethylene stream containing 20 to 80 wt. % ethylene and ethane. The reaction takes place in one of a series of series-connected reaction zones in the presence of an alkylation catalyst including a molecular sieve such as MCM-22. The temperature and pressure of the reaction zone being such that the benzene and dilute ethylene feedstock are under liquid phase conditions. The intermediate products between reaction zones are cooled and a portion of alkane, e.g., ethane, in the intermediate products is removed to maintain liquid phase by avoiding accumulation of ethane from zone to zone.
This invention relates a process for producing an alkylated aromatic compound in predominantly liquid phase alkylation reactor with an alkene feedstock containing alkene and at least 1 mol. % alkane without inter-zone alkane removal.